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Definition
The idea of plate tectonics emerged in the 1960s after new studies of the ocean floor revealed that the land masses of the Earth are actually part of much larger plates. It was discovered that the outermost layer of the Earth's crust, the lithosphere, is broken into seven pieces that we call the African, North American, South American, Eurasian, Australian, Antarctic and Pacific plates. The plates are constantly in motion and move at differing speeds and directions relative to each other.
Main Principle
The main mechanism behind plate tectonics is that the broken plates in the lithosphere move about on the layer located directly below the lithosphere, called the asthenosphere. The asthenosphere is much hotter than the lithosphere and is therefore much more fluid. Tectonic plates can move from 2 to 10 cm per year in relation to neighboring plates. As these plates move they slide against one another and slowly collide. This activity is what brings about earthquakes, volcanoes, mountains and other natural landforms.
Plate Boundaries
A plate boundary is the area on the Earth's crust where two plates meet. They are sometimes easily identified by the presence of mountain ranges, fault lines and mid-ocean ridges. There are three main types of plate boundaries: Transform, divergent and convergent boundaries. A transform boundary occurs when a plate grinds along another plate producing fault lines. Divergent boundaries occur when plates slide apart from one another and produce mid-ocean ridges and other geologic rifts. Convergent boundaries occur when two plates slide toward each other and either collide or one slides under the other (a subduction zone). These boundaries create mountain ranges, volcanoes and marine trenches.
Forces of Motion
Though motion of tectonic plates is not completely understood, it is generally accepted that the plates move because the lithosphere is much less dense than the asthenosphere beneath. This difference in density causes the lithosphere to sink into subduction zones creating a large amount of motion-generating energy. Other forces believed to cause motion of tectonic plates include "mantle dynamics" where motion is thought to be caused by friction between the currents of the asthenosphere and the lithosphere. Earth's rotation is also considered a likely candidate for causing tectonic motion in that tidal forces from gravity exerted by the Moon and Sun pulls on the Earth's crust as the Earth spins.